This project is designed to study the basic mechanisms of sugar and amino acid absorption in isolated perfused segments of rabbit proximal convoluted and proximal straight tubules. Additional studies will also involve amino acid transport in the Ehrlich ascites tumor cells with particular reference to the means energy coupling to this active transport process. The amino acids to be studied are alpha-aminoisobutyric acid (AIB) and glycine; the sugars are D-glucose and alpha-methyl-D-glucoside. By simultaneous measurement of unidirectional fluxes and intracellular concentrations of these nonelectrolytes in isolated proximal tubule segments, we plan, first, to determine the location of the active transport site(s) in these epithelia. Second, we will characterize the passive transport properties of both apic and basal-lateral membranes with respect to these nonelectrolytes. Third, we will compare the amino acid and sugar transport characteristics of superficial and juxtamedullary proximal convoluted and straight tubules. Fourth, we will determine whether bath-to-lumen "backleadk" of these nonelectrolytes involves primarily a transcellular or paracellular route. Fifth, we will examine the effects of potential modifiers of tight junction permeability, such as bathing solution oncotic pressure, cyclic AMP, Ca ions and maleic acid on the backleak permeability to amino acids and sugars. Sixth, we will continue our investigation of amino acid uptake in the Ehrlich ascites tumor cell with the objective of determining whether energy sources in addition to the Na ion electrochemical potential gradient may contribute to this active transport process.